(1) Field of the Invention:
This invention relates to a method of forming a printed circuit, more particularly to a method of forming a printed circuit by use of a specified photo-curable resin coating composition, and to the printed circuit formed therefrom.
(2) Description of the Prior Art:
The formation of the printed circuit has conventionally been carried out by a method which comprises coating a photo-curable resin coating composition on a copper foil plated insulative plate (hereinafter referred to as an insulative plate), subjecting the coated film to light exposure and development through a circuit pattern mask to form a resist film followed by melt plating or electric plating, and removing the resist film and the unnecessary part of the copper foil.
The step of forming the resist film in the aforementioned method is conventionally carried out by coating the photo-curable resin coating composition on the insulative plate with a spray coater, roll coater, curtain flow coater, or the like. However, the aforementioned step of forming the resist film has such problems that it is difficult for the photo-curable resin coating composition to be thoroughly coated on corners of through-holes when coated on the insulative plate having through-holes because the photo-curable resin coating composition is liquid at normal temperature, and that thick-coating causes defects in the film such as sag, run and the like, making it impossible to form the intended printed circuit. On the other hand, such a problem remains unsolved in that an unsaturated monomer is used in combination with the photo-curable resin to prepare the photo-curable resin coating composition in order to improve coating workability, curing characteristics and the like, and the smell and toxicity of the unsaturated monomer provide adverse effects on the coating operator from the standpoints of safety and hygiene.
In addition thereto, a process in which a coating composition by water-dispersing a photo-curable resin is subjected to electrodeposition coating, is known. However, since the electrodeposition coating composition used therein has a relatively low melting point and is prepared by dispersing a resin having a large amount of carboxyl groups in an aqueous medium containing a large amount of an alcoholic organic solvent, the film formed therefrom has poor electrical insulating properties due to such reasons that the film is thin, that volatilization of the alcoholic organic solvent is so slow that the solvent remains in the film for a long period of time, that the film contains a large amount of carboxyl groups, and so forth. Further, the aforementioned process has such a drawback that the treatment of the resulting film by electric plating, melt plating or the like in the following step causes deposition of metal on the film, resulting in making it difficult to remove the resist film with alkali or the like, and in making it impossible to form the intended printed circuit.